Polarization grating for scanning antennas



A 35mm 3 4L 3 w 5 Jar 1e 8, 1965 M. J. BOCK ETAL 3,188,642

POLARIZATION GRATING FOR SCANNING ANTENNAS Filed Aug. 26, 1959 INVENT'DRS MARVIN J. .BOCK HAROLD A. ROSE/V F/G. 6 %WM ATTOR/Vf) United States Patent 3,188,642 POLARIZATION GRATING FOR SCANNING ANTENNAS Marvin I. Bock, Woodland Hills, and Harold A. Rosen,

Santa Monica, Calif., assignors to Raytheon Company,

Lexington, Mass, a corporation of Delaware Filed Aug. 26, 1 959, Ser. No. 836,849 6 Claims. (Cl. 343-756) This invention relates to antenna systems and, more particularly, to directional antenna systems of the conical scanning type which are used in the transmission and reception of plane polarized electromagnetic energy. In directional antenna systems of the kind employing a parabolic reflector, it is frequently necessary to transmit or receive in a given plane of polarization in order to prevent energy which has become depolarized during transmission from interfering with the antenna beam pattern. Polarization selection of this type is particularly useful in connection with tri-element scanning where a rotating tri-element, such as a tri-slot or tri-pole, is rotated about its mechanical center in a polarized microwave energy field to generate an effective conical scan at three times the physical rotational velocity of the trielement radiator. A scanning device of this character is disclosed in the United States patent application of Paul S. Miller, Serial No. 656,474, filed April 26, 1957, and assigned to the assignee of this application. In this device, the rotating tri slot element senses, during rotation, energy polarized in a plurality of planes and, in particular, energy polarized at 45 degrees due to the three to-one ratio of electrical-to-mechanical scan. This is particularly true since the amplitude and phase of the scanning modulation of a tri-element radiator depends, in part, upon the polarization angle of the received energy. This dependence results in cross-coupling of horizontal and vertical components of polarized energy during the scanning process. This, in turn, changes the beam pattern whenever the polarization angle of the incoming electromagnetic energy differs from that for which the antenna is adjusted. In numerous applications, therefore, it would be desirable to provide an external grating or filter which removes undesirable cross-polarized energy components before the scanning process occurs.

In accordance with the polarization filter of the invention, a conic-shaped grating of low-loss material is formed from a plurality of parallel strips, alternately, of conductive material and dielectric material, said conductive strips being bonded to the dielectric material. When the polarization dish is illuminated with electromagnetic energy of random polarization, the parallel metallic strips act as a reflector to the unwanted cross-polarized energy, that is, polarizations substantially parallel to the strips. However, energy which is polarized perpendicular to the strips passes through the polarization grating. Whether the energy illuminating the polarized grating emanates from a rotating tri-slot radiator of the direct transmission type as in the aforementioned Miller patent application or from a rotating parasitic tri-element of the indirect transmission type, the grating operates as a selective filter which rejects undesirable components of polarized energy.

Further objects and advantages of this invention will be more apparent as the description progresses, reference being made to the accompanying drawing wherein:

FIG. 1 is a side view, partly in section, of an antenna structure embodying the principles of the present invention;

FIG. 2 is a rear elevation of a portion of the polarization grating shown in FIG. 1;

FIG. 3 is a section taken along the line 33 of FIG. 2;

3,188,642 Patented June 8, 1965 FIG. 4 is a front view of the rotary tri-slot disc which is used in the antenna system of FIG. 1;

FIG. 5 is a front view of the rotary tri-pole element, which is also used in the antenna system of FIG. 1;

FIG. 6 is a side elevation of another embodiment of the invention; and

FIG. 7 is a front view of the rotary tri slot disc which is directly fed by the circular waveguide section shown in FIG. 6.

Referring now to FIG. 1 there is shown a polarization grating 10 used in connection with a tri-element radiator of the direct transmission type described in the copending application of Jesse L. Butler, Serial No. 457,072, filed September 17, 1954, now United States Letters Patent No. 2,895,131, issued July 14, 1959 and assigned to the assignee of this application. The polarization selective grating is applicable to both transmitting and receiving antenna systems. In the present embodiment, the polarization grating 10 encloses, the end portion of parabolic reflector 12. The reflector 12 is illuminated by electromagnetic energy reflecting from a rotary tripole or tri-slot element 14. This electromagnetic energy is fed from a waveguide feed section 16. More particularly, the polarization grating 10 as shown in FIGS. 2 and 3, comprises a plurality of parallel strips of metallic material 18, such as for example aluminum foil. The strips are positioned side by side with their corresponding edges parallel to each other. The space between the strips contains a low-loss dielectric material which consists of a foam chemical having a dielectric constant approaching that of air. The layers or rectangular slats of dielectric material 20, as shown in the present embodiment, are approximately one quarter-inch thick and one half-inch wide. Each aluminum foil strip 18 is bonded to the corresponding layer of dielectric material 20 by a cement. A fiber glass mounting ring 22 is cemented circumferentially to the grating to provide a lightweight supporting structure. This supporting structure, in turn, is screw-mounted to the parabolic reflector 12 as shown in FIG. 1. It should be understood that the dielectric slats 20 can consist of any dielectric foam of a dielectric constant of approximately 1.1 and 1.2 at the operating frequency of the antenna system. For example, a dielectric material such as the well-known Hycar can be used. The rectangular slats are cut from a solid block of dielectric foam, a strip of aluminum foil is cemented between each slat, and the conical antenna configuration is then milled out of the cemented block.

A typical dielectric foam similar to Hycar is produced by mixing a liquid isocyanate and a polyester resin in approximately equal amounts. This mix is then poured into a closed mold and permitted to expand to a density of approximately ten pounds per cubic foot. After bak ing the liquid material in the mold at a temperature of approximately 250 degrees Fahrenheit for three hours, the dielectric foam is then permitted to cool to room temperature.

Referring again to FIG. 1, the waveguide feed section 16 comp-rises a rectangular waveguide 24 for producing a fixed polarization of energy passing there-through and an input flange 25. The rectangular waveguide 24 tapers into a circular waveguide section 26, as shown at 27. The circular waveguide section 26 contains a low-loss dielectric material 30, which acts as a polyrod feed for the parasitic scanning disc 14. This materal is inserted into the waveguide and bonded thereto. The end portion of the dielectric material 30 transfers energy from the waveguide section to the tri-slot element 14, which in turn, illuminates the parabolic reflector 12. The rotary tri-element 14 may consist of a tri-slot disc as shown in FIG. 4 or, alternatively, a tri-pole disc 15, as shown in FIG. 5.

The rotary tri-element 14 is rotated by means of a motor-generator 34 which is held firmly by a clamp 35 to which is attached four support brackets 36. These brackets are, in turn, attached to the parabolic reflector 12, thereby maintaining the position of the tri-element radiator at approximately the focal point of the parabolic reflector.

As shown, the motor-generator :34 is a special device containing a 1,000-cycle synchronous motor and a 250- cycle generator mounted concentrically in a common case. The motor portion of the motor-generator 22 is used primarily for rotating the tri-element disc 14, and the generator portion is used to provide an A.C. reference voltage for determining the position of the tri-element disc 14 at any specific time. This A.C. generated reference voltage is necessary when it is considered that the threeold scan rate is accomplished by actually having three rotating radial slots. It is necessary, therefore, to know at any particular time which slot is receiving or transmitting energy, the problem being analogous to a lobing antenna where there are three loges to be identified. The motor-generator is of suflicient size to fit within the centrally located aperature in polarization grating "10. In operation as an energy transmitter, therefore, plane polarized energy is fed toward the rotating tri-element 14 and, in turn, illuminates parabolic reflector 12. The reflected energy is directed toward the polarization grating with the desired polarization passing therethrough and thence into space. in this manner, the polarization grating transmits energy in the desired plane while the remaining components of the energy are rejected by the horizonal parallel metallic strips. However, in operation as a receiving antenna, the undesired polarization components are prevented by the grating from entering the antenna system.

Referring now to FIG. 6, the polarization filter is used in connection with a tri-slot radiator of the front transmission type feed, described in detail in the aforementioned Miller application. Here, the motor-generator 34 is mounted to the rear of the parobolic reflector 12. A rotating slotted plate or disc 44 as shown in FIG. 7, is mounted in the in portion of the circular waveguide sec tion 45, and is provided with a rotary joint 46 to permit rotation of the slotted disc while the circular waveguide portion 45 is held fixed. The slotted disc 44 has three-half wave length slots 41, 42 and 43, circumferentially mounted around the center of rotation of the disc. These circumfertial slots are directly excited and the offset center of radiation is a direct function of the distance between the feed center and the center of the slots. Generally, the greater the distance from the feed center to the center of the slots, the greater is the radiation offset. The tri-slot metal disc in this embodiment can be constructed in a variety of shapes and forms, the only limitation being that the three circumferential slots are so constructed and positioned as to make the slots concentric about -an axis passing through their center point, and at the same time extend circumferentially about the center point of the metal disc, thereby being symmetrical about the axis of rotation.

In order to rotate the slotted disc 44, a shaft or rod 56 is connected to the shaft of the motor-generator 34 by means of a set screw 47. This shaft 56 extends through an aperture, not shown, at the midpoint of the parabolic reflector 12. Energy is fed by way of flange 48 into the rectangular waveguide section 49 which tapers into circular waveguide section 45, as shown at 50. The parabolic reflector 12 is held rigidly to the rectangular waveguide 49 by means of a bracket 52 which is connected to a metal boss 54 attached to the reflector 12. In this manner, the illuminated parabolic reflector 12 reflects energy through the polarization grating 10 in the desired plane while the remaining components of energy are selectively filtered out of the beam pattern.

As described in detail in the aforementioned Miller patent application, the power radiated by a given slot depends upon its orientation relative to the dominant E-vector of the feed guide. For example, as shown in FIG. 7, when slot 41 is centered about the vertical axis it receives more excitation than either slot 42 or 43 due to its orientation with respect to the vertically polarized E-vector. Accordingly the left-right effects of slots 42 and 43 cancel and a resulting center of radiation in the up direction is obtained. In the second position, however, with slot 41 rotated 30 degrees with respect to the vertical axis, as shown in FIG. 7, slot 43 may be considered inactive relative to the E-vector, and the resulting effects of slots 41 and 42 produce an offset center of radiation to the right, which produces a -degree rotation of the center of radiation for a 30-degree rotation of the tri-slot disc. In like manner, if slot 41 is rotated to 60 degrees from the vertical axis, slots 41 and 42 are excited to a substantially lesser degree than slot 41. Thus, a downward displacement of the center of radiation is produced due to the greater excitation of slot 42 because of its favorable orientation with respect to the vertical E-vector. A rotation of degrees of the effective center of radiation is therby obtained for a 60-degree mechanical rotation of the triaslot disc. In this manner, the scanning antenna system shown in FIG. 6 produces a 3-to-1 ratio of electrical-to-mechanical scan by rotating the disc 44 in the path of microwave energy of fixed polarization propagating through the circular waveguide 45. In other words, the tri-element or tri-pole scanning devices produce an offset center radiation which describes a perfect circle about the mechanical center of rotation at a rate which is three times the rate of mechanical rotation. However, the radiation offset center of the disc 14 of FIG. 4 is in the opposite direction from that obtained with the tri-slot disc of FIG. 7. This is because the vertically-positioned slot in FIG. 4 receives minimum excitation due to its orientation parallel to the vertical E-vector.

It should be understood that numerous embodiments of the polarization selective filter grating can be used in connection with these tri-scan antennas or with any trielement antenna depending upon the percentage of reuected radiation permitted to enter the antenna beam. Thus, a polarizatioin filter can be used which permits only the desired component of the cross-polarized energy to leave or enter respectively, a transmitting or receiving conical scanning tri-element antenna system.

This completes the description and the embodiments of the invention illustrated herein. However, many modifications of the advantages therof will be apparent to persons skilled in the art without departing from the spirit and scope of this invention. Accordingly, it is desired that this invention not be limited to the particular details of the embodiments disclosed herein, except as defined in the appended claims.

What is claimed is:

1. In combination, a polarization grating for electromagnetic energy having a fixed polarization comprising a plurality of parallel strips alternately of conductive material and dielectric material, the widths of said conduc-- tive strips being substantially less than the widths of said dielectric strips, the thickness of each conductive strip being small with respect to its width, an electromagnetic energy radiating element positioned adjacent to said polarization grating, said radiating element comprising a rotatable disc having three apertures symmetrically disposed in a plane perpendicular to the axis of rotation, means for rotating said rotatable disc about said axis, means for exciting said apertures with electromagnetic energy having a fixed polarization, and means for reflecting electromagnetic energy from said apertures toward said polarization grating.

2. In combination, a polarization grating for filtering electromagnetic energy comprising a plurality of plates of metallic material, said plates being positioned side by side with their corresponding edges parallel to each other, the space between said plates containing a low-loss dielectric material for the passage of electromagnetic energy polarized in a direction perpendicular to the plane of the plates, an electromagnetic energy radiating element positioned adjacent to said polarization grating, said radiating element comprising a rotatable disc having three apertures symmetrically disposed in a plane perpendicular to the axis of rotation, means for rotating said rotatable disc about said axis, and means for exciting said apertures with electromagnetic energy having a fixed polarization.

3. In combination, a polarization grating for inhibiting electromagnetic energy having a predetermined direction of polarization comprising a plurality of plates of metallic material, said plates being positioned side by side With their corresponding edges parallel to each other, the space between said plates containing a low-loss dielectric material for the passage of electromagnetic energy polarized in a direction perpendicular to the plane of the plates, a tri-scan antenna positioned adjacent to said polarization grating, said tri-scan antenna including three radiating elements symmetrically disposed substantially in a plane perpendicular to an axis of rotation, means for rotating said radiating elements about said axis, means for exciting said radiating elements with electromagnetic energy, and reflecting means in register with said polarization grating.

4. In combination, a polarization grating for electromagnetic energy comprising a plurality of strips of conductive material arranged parallel to each other, the space between said strips containing a low-loss dielectric material for the passage of electromagnetic energy polarized in a direction perpendicular to the plane of said strips, a section of waveguide, a rotatable metal plate having three slots symmetrically disposed in a plane perpendicular to the axis of rotation, said metal plate positioned at one end of said section of waveguide and perpendicular to the longitudinal axis thereof, means for rotating said metal plate, means for propagating electromagnetic energy having a fixed polarization along said Waveguide and through said slots, and a reflector adapted to direct said electromagnetic energy propagated from said slots toward said polarization grating.

5. In combination, a polarization grating for electromagnetic energy comprising a plurality of strips of conductive material arranged parallel to each other, the space between said strips containing a low-loss dielectric material for the passage of electromagnetic energy polarized in a direction perpendicular to the plane of said strips, a section of waveguide, a rotatable metal plate having three slots symmetrically disposed in a plane perpendicular to the axis of rotation, said metal plate posi tioned at one end of said section of waveguide and perpendicular to the longitudinal axis thereof, means for rotating said metal plate, means for propagating electromagnetic energy having a fixed polarization along said waveguide and through said slots, and a parabolic reflector in register with said polarization grating for directing toward said grating the electromagnetic energy propagated from said slots.

6. In combination, a polarization grating for electromagnetic energy comprising a plurality of strips of conductive material arranged parallel to each other, the space between said strips containing a low-loss dielectric material for the passage of electromagnetic energy po larized in a direction perpendicular to the plane of said strips, a section of waveguide, a rotatable metal plate having three elements symmetrically disposed in a plane perpendicular to the axis of rotation, said metal plate positioned at one end of said section of waveguide and perpendicular to the longitudinal axis thereof, means for rotating said metal plate, means for propagating electromagnetic energy having a fixed polarization along said waveguide and toward said elements, and a parabolic reflector adapted to direct said electromagnetic energy through said polarization grating positioned perpendicular to the path of reflected electromagnetic energy.

References Cited by the Examiner UNITED STATES PATENTS 2,607,009 8/52 Aifel 343756 X 2,753,551 7/56 Richmond 343756 X 2,818,564 12/57 Butler 343756 2,822,540 2/58 Butler 343756 X 2,864,083 12/58 Butler 343-756 2,895,131 7/59 Butler 343754 3,119,109 1/64 Miller 'et al 343-756 ELI LIEBERMAN, Primary Examiner.

FREDERICK M. STRADER, CHESTER L. JUSTUS,

KATHLEEN H, CLAFFY, Examiners.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,188,642 June 8, 1965 Marvin J. Bock et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column line 12, for "direct" read indirect line 42, for "and" read to line 65, for "materal read material column 3, line 21, for "loges" read lobes line 29, for "in" read In line 48, for "circumfertial" read circumferential column 4, line 30, after "center" insert of lines 42 and 43, for "reuectedfl'aread'-- reflected Signed and sealed this 15th day of February 1966.

( L) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. IN COMBINATION, A POLARIZATION GRATING FOR ELECTROMAGNETIC ENERGY HAVING A FIXED POLARIZATION COMPRISING A PLURALITY OF PARALLEL STRIPS ALTERNATELY OF CONDUCTIVE MATERIAL AND DIELECTRIC MATERIAL, THE WIDTHS OF SAID CONDUCTIVE STRIPS BEING SUBSTANTIALLY LESS THAN THE WIDTHS OF SAID DIELECTRIC STRIPS, THE THICKNESS OF EACH CONDUCTIVE STRIP BEING SMALL WITH RESPECT TO ITS WIDTH, AN ELECTROMAGNETIC ENERGY RADIATING ELEMENT POSITIONED ADJACENT TO SAID POLARIZATION GRATING, SAID RADIATING ELEMENT COMPRISING A ROTATABLE DISC HAVING THREE APERTURES SYMMETRICALLY DISPOSED IN A PLANE PERPENDICULAR TO THE AXIS OF ROTATION, MEANS FOR ROTATING SAID ROTATABLE DISC ABOUT SAID AXIS, MEANS FOR EXCITING SAID APERTURES WITH ELECTROMAGNETIC ENERGY HAVING A FIXED POLARIZATION, AND MEANS FOR REFLECTING EECTROMAGNETIC ENERGY FROM SAID APERTURES TOWARD SAID POLARIZATION GRATING. 